Fix upload cancel in e2e rooms
This is a bit of a mess of passing promises around - we weren't
taking the right promise to pass to cancelUpload.
Also e2e uploads take time to read into memory & encrypt, so allow
cancelling them during those phases too, even though we can't abort
those phases before they're done - we do mark the upload as cancelled
though so filter the current uploads for cancelled ones.
Fixes https://github.com/vector-im/riot-web/issues/4891
